KAP 140 trouble

The B/K KAP 140 autopilot in my 2006 Cessna T-182T has a problem that is
baffling the avionics shop. The altitude capture/hold function does not work.
Here are the symptoms:

o All other functions work, including +/- vertical speed control.

o It will accept and correctly execute a vertical speed setting, but will
fly right through the target altitude.

o If Altitude Hold is engaged in level flight, it will not maintain
altitude.

o If Vertical Speed is engaged in level flight and set to "0", it will do
a fair job of maintaining altitude.

o According to the tech, the KAP 140 perpetually believes it is at 13,000
ft, and nothing will convince it otherwise. The unit has its own barometric
setting, independent of the G1000. However, the latest theory at the shop is
that it is a Garmin problem.


The tech's have spent a lot of time on the phone with Garmin and B/K,
listening to a lot of finger-pointing back and forth. They currently have
over 20 hours labor charged to the job--thank goodness it's warranty. I'm
able to fly the plane this weekend, but it goes back in the shop Monday.

Anybody have any ideas?

--
Dan

"You spend money to buy aggravation."
-Chinese proverb

"Peter Clark" wrote:

>
> Sounds like the AP computer. Did the code download come up with any
> codes?

Don't know.

> The shop did hook a laptop up to the KAP140, right?

Yes.


> There is no connection between the G1000 and the KAP140 outside of the
> GPSS and HDG outputs. Altitude modes are all internal to the KAP140
> which is why it's got a static input and you need to enter the
> altitude (again) and baro (again) when making changes.

That's what I thought. I don't see how the G1000 could have anything to do
with it.



> Is there a blockage in the static line heading towards the KAP140?

Evidently not.

> Any other wierd readings on the primary or secondary instruments?

Nope, all fine.

Thanks for your input. I'm looking for help asking questons when I see the
avionics shop guys tomorrow.

--
Dan
T-182T at BFM

"Dan Luke" wrote:

>
> Anybody have any ideas?


John Jones, a fellow Cessna Pilots 'n member, has put me on the right
track, I believe. The autopilot receives its altitude information via Gray
code from the Garmin transponder. If a particular line is grounded, it would
hold the autopilot at 13,000 ft.

Tomorrow I'll take this information to the avionics shop and see what they
say.

--
Dan
T-182T at BFM

> John Jones, a fellow Cessna Pilots 'n member, has put me on the right
> track, I believe. The autopilot receives its altitude information via Gray
> code from the Garmin transponder. If a particular line is grounded, it
> would hold the autopilot at 13,000 ft.
>
> Tomorrow I'll take this information to the avionics shop and see what they
> say.

That wasn't it. The incorrect altitude Gray code is definitely coming from
the Garmin side.

"Peter Clark" wrote:

>
> Perhaps to complete my reasoning - the original post says it drifts
> off altitude when in level flight clicking

Sorry, that was incorrect. If ALT is selected in level flight, it will hold.
Altitude capture is the only thing not working.


> AFAIK the only time
> the KAP140 cares about the altimeter setting and the preselected
> altitude is when it's in a capture mode, and it uses it's internal
> pressure transducer to maintain altitude. Course, wouldn't be the
> first time I've been wrong about the internal workings of the box, but
> it's really not that big a deal to replace it and see - it's not like
> there's no known history of the KAP140 computers being problematic...

The KAP 140 has been swapped for a known good one. No joy.

The wiring has been checked for a ground on the Gray code lines & found to be
ok.

The tech has plugged a laptop into the system and sent various coded
altitudes to the KAP 140 and it has decoded them correctly.

Thanks a bunch for your input.

--
Dan
T-182T at BFM

"Peter Clark" wrote:


> Have you tried changing the baro in flight to some number of inches
> off the real pressure to see just how far the system is mis-reading
> the baro? That will tell you something.

Yes. On the ground I have set the xponder to squawk altitude and turned the
altitude preset on the KAP 140 up until it gives the "1,000 ft. before"
warning. That happens at 13,000 ft.
>
> I'll have to go pull my wiring diagrams out since I don't recall off
> the top of my head, but does the KAP140 actually get it's graycode
> from the GTX, or does it get it from one of the outputs on GIA#2? Have
> they tried swapping GIA#1 and GIA#2 to see if there is any difference?

The GIA2. The GIAs have been swapped and the problem stays.

--
Dan
T-182T at BFM

Dan Luke wrote:
> The GIA2. The GIAs have been swapped and the problem stays.

Is the previous owner paying for all this repair time?

"Peter Clark" wrote:

>>
>>The GIA2. The GIAs have been swapped and the problem stays.
>
> Sounds like at this point they're going to have to build a harness and
> ring out the graycode lines end-to-end from the GIA2 output through to
> the AP computer connector individually. If both GIAs are producing
> the issue and two AP computers are doing it, there's got to be
> something going wrong in the harness. It's all that's left..

Seems logical to me.

The tech says he is talking to Garmin about the possibility that it is a
software problem, although I cannot imagine how such a thing could appear in
only one of many identical airplanes. Faulty memory chip, maybe?

--
Dan
T-182T at BFM

"Peter Clark" wrote:

> Have they tried
> hooking up the pitot/staic testset and pumping it up?

Don't know. They're getting complete schematics from Garmin for further
trouble shooting.

--
Dan
T-182T at BFM

"Peter Clark" wrote:

>>Don't know. They're getting complete schematics from Garmin for further
>>trouble shooting.
>
> The baseline schematics from Garmin won't neccessarily match how
> Cessna has it set up. They should be looking at the wiring diagrams
> on the Cessna maintenance manual CD. Have they called Cessna about
> this?

That's the same thing John Jones on the CPA forum said.

I'll ask them tomorrow.

--
Dan
T-182T at BFM